diff --git a/cmake/gitcommit.cmake b/cmake/gitcommit.cmake
index 76753aa7..2be90292 100644
--- a/cmake/gitcommit.cmake
+++ b/cmake/gitcommit.cmake
@@ -37,7 +37,8 @@ else()
   endif()
 endif()
 
-file(WRITE "${OUTPUT_FILE}" "\
+if(OUTPUT_TYPE STREQUAL header)
+  file(WRITE "${OUTPUT_FILE}" "\
 /*
  * cmake_commit.h - string literal giving the source git commit, if known.
  *
@@ -46,3 +47,6 @@ file(WRITE "${OUTPUT_FILE}" "\
 
 const char commitid[] = \"${commit}\";
 ")
+else()
+  message(FATAL_ERROR "Set OUTPUT_TYPE when running this script")
+endif()
diff --git a/cmake/setup.cmake b/cmake/setup.cmake
index ffc23184..8b64acc8 100644
--- a/cmake/setup.cmake
+++ b/cmake/setup.cmake
@@ -44,6 +44,7 @@ add_custom_target(check_git_commit
     -DGIT_EXECUTABLE=${GIT_EXECUTABLE}
     -DTOPLEVEL_SOURCE_DIR=${CMAKE_SOURCE_DIR}
     -DOUTPUT_FILE=${INTERMEDIATE_COMMIT_C}
+    -DOUTPUT_TYPE=header
     -P ${CMAKE_SOURCE_DIR}/cmake/gitcommit.cmake
   DEPENDS ${CMAKE_SOURCE_DIR}/cmake/gitcommit.cmake
   WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}